Insulin stimulates phosphorylation of both Alpha and Beta-subunits of its own receptor in a cell-free system using a solubilized, partially purified preparation of insulin receptors from rat liver membranes. The receptor subunits are isolated by specific immunoprecipitation with anti-receptor antibodies followed by SDS-gel electrophoresis. Under reduced conditions two bands, 95K and 135K, were selectively labeled which correspond exactly to the position of the Alpha and Beta subunits of the insulin receptor. Phosphorylation of both subunits was stimulated 4-6 fold with pork insulin. Guinea Pig insulin was about 2% as potent as pork insulin, whereas EGF, ACTH, and glucagon were ineffective. This process suggests that phosphorylation is an early step in insulin action.